Improvements to UI: Make Plinth look like a portal rather than Admin UI

Created by: SunilMohanAdapa

These changes are in addition to dashboard proposal in #20 .

Inline with the needs for Plinth to become the front page for FreedomBox, we could improve the Plinth UI. I suggest getting a much 'lighter' feel to the Plinth interface and also focus on more important things (Apps) and de-emphasizing the rarely used things (Configuration and Help).

When combined with Dashboard changes, this will be much more usable as a portal.